1. Field of the Invention
The present invention relates to a sensor or accelerometer of the type having a cantilevered beam with a free end at which an inertial mass is mounted, the beam having a piezoelectric layer and a supporting layer and the inertial mass being located eccentrically relative to the longitudinal direction of the beam.
2. Description of the Prior Art
PCT Application WO 89/00294 discloses a conventional beam type accelerometer of the above kind which comprises two layers, a supporting layer made of silicon and a piezoelectric layer bonded to said layer. This accelerometer is intended to be suitable for mass manufacture from silicon wafers and is also intended to be sensitive in a single plane only relative to its mounting. The accelerometer also is intended to have improved sensitivity.
Although this known accelerometer is relatively easy to manufacture, it is sensitive in a plane and it is relatively large, the preferred embodiment having an overall length of about 4 mm.
U.S. Pat. No. 5,425,750 discloses another accelerometer comprising a layered beam. The beam comprises a electrically conducting substrate such as beryllium copper which on each side is covered with a transducing layer of a piezoelectric polymer. The accelerometer is provided with an additional sensing mass in the free end of the beam which is offset in relation to the longitudinal direction of the beam in order to ensure that the accelerometer is sensitive in all directions in a plane.
This known accelerometer thus also is sensitive in several directions, particularly in a direction which is oriented along the longitudinal extent of the beam, i. e. in a direction which is oriented orthogonally to the main direction of sensitivity. The device further is relatively large (a length of about 4 mm in the preferred embodiment) and is comparatively complicated to manufacture.
In many applications, such as for instance so-called rate responsive pacemakers, i. e. pacemakers sensing the physical activity of the patient in order to increase or decrease the stimulation rate applied to the heart in dependence on said activity, there is a need of an accelerometer which is easy and inexpensive to manufacture, which, if so is desired, may have one main sensor sensitivity axis and a negligible cross sensitivity, i. e. a sensitivity which is negligible in directions orthogonal to said main sensitivity axis, which is made small, which is designed to be very sensitive and which may be designed to be highly resistant to shocks.